XUL (XML User Interface Language) is a programming language that uses a combination of existing technologies like JavaScript, CSS (Cascading Style Sheets), and XML (eXtensible Markup Language) to create user interfaces for applications. XUL is a markup language developed by Mozilla for building cross-platform applications, and it allows developers to create rich user interfaces using familiar web technologies. JavaScript is used for adding interactivity and dynamic behavior to XUL-based applications, CSS is used for styling and layout, and XML is used for defining the structure of the user interface components. XUL is commonly used in Mozilla applications like Firefox and Thunderbird for creating user interfaces for extensions and add-ons.